pgsql: Enable -Wimplicit-fallthrough option for clang

Enable -Wimplicit-fallthrough option for clang

On clang, -Wimplicit-fallthrough requires annotations with attributes,
but on gcc, -Wimplicit-fallthrough is the same as
-Wimplicit-fallthrough=3, which allows annotations with comments. In
order to enforce consistent annotations with attributes on both
compilers, we test first for -Wimplicit-fallthrough=5, which will
succeed on gcc, and if that is not found we test for
-Wimplicit-fallthrough.

Reviewed-by: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Discussion: https://www.postgresql.org/message-id/flat/76a8efcd-925a-4eaf-bdd1-d972cd1a32ff%40eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0284e07599a53d27d744042c10aec3e2e674d5b5

Modified Files
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configure | 103 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
configure.ac | 14 ++++++++
meson.build | 19 ++++++++++-
3 files changed, 135 insertions(+), 1 deletion(-)